John Carmichael wrote: > So Mark, which of your three different averages should one use as the > average amount a longitudinally corrected is "off" (either positive or > negative) on any randomly selected day of the year. Is the correct answer > your mean absolute error of 7.2 minutes? Is this what I should tell my > sundial customers?
The precise answer, unfortunately, is more complicated. If your dial face can "see" the Sun at every daylight moment throughout the year (a horizontal dial, for example), and lies on the equator (where there is practically no seasonal variation in the length of day), then the mean amount by which the dial is "off" is 7.2 minutes. My Seattle paper discussed how the various mean errors change with latitude and dial orientation. Take the extreme case of a horizontal dial at the North Pole. From that geographic location, only the upper half of the analemma is visible. And because the analemma's northern "lobe" is narrower than the southern "lobe," the mean absolute error at latitude +90 degrees is only 3.5 minutes -- less than half the mean absolute error of horizontal dials on the equator. A horizontal at the South Pole -- which only sees the lower, fatter part of the analemma -- would exhibit a mean absolute error of 11.0 minutes. Therefore a polar bear's sundial is in closer agreement, on average, with wristwatch time than a penguin's. For latitude +32 (isn't that your approximate latitude, John?), a horizontal dial's mean absolute error would be 6.8 minutes. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Mark Gingrich [EMAIL PROTECTED] San Leandro, California
